Gas turbine engines derive their thrust or other power output by the combustion of fuels. Since engine power and economy both improve with increased temperature, there has been a persistent trend in the gas turbine engine field toward increased engine operating temperatures. For many years this trend was accommodated by the development of improved materials. Whereas early gas turbine engines were based mainly on alloys derived from common steels, the modern gas turbine engine relies on nickel and cobalt base superalloys in many critical applications. It appears for the moment that property limits for metallic materials are being approached or perhaps have been reached, but the demand for increased temperature capability continues. While work is underway to develop ceramic turbine materials, this work is at a very preliminary stage and many difficulties must be overcome before ceramics play a structural role in gas turbine engines.
Not surprisingly, attempts have been made to use ceramics as coating materials to provide thermal insulation to metallic substrates and thereby permit increased engine operating temperature without substrate damage. Such attempts have met with a certain degree of success as described, nonetheless, the durability of ceramic thermal barrier coatings remains a concern because such coatings are used in man rated applications and safety considerations require maximum durability. The basic approach which has generally been taken is to apply an oxidation resistant metallic bond coat to the substrate and then to apply to this bond coat a ceramic coating, or in some cases, a mixed metal ceramic coating. Several patents have suggested the use of MCrAlY materials for the bond coat. MCrAlY materials were developed for the protective coating of metallic components to protect them from oxidation and corrosion under high temperature conditions. Such MCrAlY coatings are described, for example, in U.S. Pat. Nos. 3,676,085, 3,928,026 and 4,585,481.
The currently favored ceramic constituent is zirconia, but because zirconia undergoes a phase transformation at about 1800.degree. F., it is necessary to make additions to the zirconia to provide a stable or at least controlled microstructure at increasing temperature.
Patents which appear particularly pertinent to this subject area include U.S. Pat. No. 4,055,705 which suggests a thermal barrier coating system using a NiCrAlY bond coat and a zirconia based ceramic coating which may contain, for example, 12% yttria for stabilization. U.S. Pat. No. 4,248,940, which shares a common assignee with the present application, describes a similar thermal barrier coating, but with emphasis on the type of thermal barrier coating in which the composition of the coating is graded from 100% metal at the bond coat to 100% ceramic at the outer surface. This patent describes the use of MCrAlY bond coats, including NiCoCrAlY, and mentions the use of yttria stabilized zirconia. U.S. Pat. No. 4,328,285 describes a ceramic thermal barrier coating using a CoCrAlY or NiCrAlY bond coat with ceria stabilized zirconia. U.S. Pat. No. 4,335,190 describes a thermal barrier coating in which a NiCrAlY or CoCrAlY bond coat has a sputtered coating of yttria stabilized zirconia on which is plasma sprayed a further coating of yttria stabilized zirconia. U.S. Pat. No. 4,402,992 describes a method for applying a ceramic thermal barrier coating to hollow turbine hardware containing cooling holes without blockage of the holes. The specifics of the coating mentioned are a NiCrAlY or a CoCrAly bond coat with yttria stabilized zirconia. U.S. Pat. No. 4,457,948 describes a method for producing a favorable crack pattern in a ceramic thermal barrier coating to enhance its durability. The coating mentioned has a NiCrAlY bond coat and a fully yttria stabilized zirconia coating. U.S. Pat. No. 4,481,151 describes another ceramic thermal barrier coating in which the bond coat comprises NiCrAlY or CoCrAlY, but wherein the yttrium constituent may be replaced by ytterbium. The ceramic constituent is partially yttria or ytterbium stabilized zirconia. U.S. Pat. No. 4,535,033 is a continuation-in-part application of the previously mentioned U.S. Pat. No. 4,4481,151 and deals with a ceramic thermal barrier coating in which zirconia is stabilized by ytterbia.